If the Romulans decided to send a few hundred cloaked warbirds to attack Earth, can they be detected upon crossing the neutral zone? Does Starfleet have any special sensor stations to detect/track a cloaked ship?
Secondly what are the defences around Earth and the solar system?
If you're talking about the 24th century, cloaking technology is quite advanced at this point, but there are still some limited defenses.
During DS9: "Visionary", Jadzia Dax scanned the nearby space around the station out to a radius of 2 km. She didn't find anything at first, but then she began to focus on the lower subspace bandwidths and noticed traces of low level tetryon emissions. The emissions appeared to be orbiting the station in an elliptical path, and since there were no neutron stars nearby, the DS9 crew deduced that it must be coming from the quantum singularity of a cloaked Romulan warbird.
Later on, this same technique was used by DS9 to detect a large cloaked joint Romulan-Cardassian fleet entering the wormhole to attack the Dominion.
Other active-scanning techniques have been used effectively against Romulan cloaks, such as the Federation's tachyon detection grid. However, this detection method requires that the cloaked ship travel through (thus "tripping") the tachyon field generated by an active tachyon beam being transmitted between two ships or sensing stations. The single-ship implementation of this is the tachyon scan, which has a much more limited range.
Another known active-scanning technique uses antiproton beams. Romulan (and perhaps other) cloaking devices typically leave behind a trail of risidual antiprotons, which can be picked up using an antiproton beam. This is how the Jem'Hadar detect cloaked ships (including the Defiant)
However, advanced ships like the Scimitar emit neither tachyons nor antiprotons and are thus impervious to all tachyon- or antiproton-based detection techniques. Luckily, the Scimitar appears to be a one-of-a-kind prototype, though the Remans surely retained this technology and could possibly deploy it once again at some point (then again, the Klingons never did manage to mass produce Chang's unique cloaking device once the prototype was destroyed).
The Star Trek: Star Charts show that the Federation-Romulan neutral zone is the most heavily defended Federation border in the 24th century. There are 44 Federation outposts/space stations (Romeo Outposts, Sierra Outposts, Quebec Outposts, Tango Outposts and Earth Outposts) lining the neutral zone, in addition to 6 star bases. To reach Earth, the Romulan fleet would need to pass through not just the gravitic sensor net, but it would have to get past tachyon scans by Federation patrols as well as Federation outposts, starbases, and the protected space of Federation member planets/systems:
Any massed formation of dozens or more warbirds would easily be detected on long-range sensors by Federation border outposts even before they crossed the neutral zone and would be engaged long before they reached Earth. The Romulans would not just be waging war against Earth but against the Federation and all of its allies, most notably the Klingons. Between the Federation and the Klingons, the Romulans are effectively flanked from all sides, with only a 50 light-year gap that prevents them from being completely surrounded.
Most likely what would happen is that Federation outposts would detect the large Romulan fleet massing on the other side of the neutral zone by their enormous tetryon signature. At this point, the element of surprise would be lost and the cloaks would only be good for avoiding the targeting scanners of intercepting vessels.
Starfleet would send its response fleet to intercept the Romulans and also deploy a tachyon detection grid if the Romulan fleet disperses into smaller 2-3 ship elements to try to evade detection. All Federation science stations and military bases would also be put on high alert, making tachyon sweeps for cloaked ships.
A communique would be sent to Qo'noS, as the Klingon High Council would surely want to launch its own offensive against their, now very vulnerable, sworn enemies. Like the Red Army during WW2, the Klingons would open up a new front on the opposite side of the Romulan Star Empire while Humans, Vulcans, Andorians, Xindi, Tellarites, Trill, Benzites, etc. (and possibly even Ferengi) joined together to attack the main Romulan fleet.
If any remnants of the Romulan attack force reaches Sector 001, they'd be engaged by starships assigned to local patrol as well as reinforcements from nearby sectors like Deneb, Denobula and Andoria. As the Romulan fleet approaches the Sol system, starships and attack fighters would be scrambled from Jupiter Station and the Mars colonies. Then the Romulan ships would need to penetrate the Mars Defense Perimeter sentry pods. And as the last line of defense, the Earth has the Global Security Net, which consistent of unknown planetary defenses but probably includes some orbital weapons and more attack fighters.
Either way, it would be more or less a one-way trip for every Romulan warbird involved and a suicidal move by the Romulan Empire. The Romulans only ever had probably less than 500 D'Deridex heavy cruisers at any time, and that number was reduced to less than 200 by the end of the Dominion War. Even at full strength, committing "hundreds" of warbirds to an incursion into Federation space would leave their empire completely defenseless against the Klingons and any other neighboring powers. Unwilling members of the Star Empire would soon rebel as well, further weakening the Romulans until the entire empire collapsed.
And it's extremely unlikely they would succeed where the Borg and Dominion have failed. So they would effectively achieve nothing and lose everything.
The federation protects their borders using Gravitic Sensor Nets to detect cloaked ships attempting to cross the border
It was not considered completely effective, and was eventually supplemented by the tachyon detection grid.
From the top of my head:
At least as far as the TV shows and movies go there is a lot of inconsistency with tracking cloaked ships. Apparently space drives in Star Trek create something trackable ... mostly neutrino emissions IIRC. Therefore you can track about any moving cloaked ship, as long as you look for residual radiation emitting from its engines. This should make anything as big as a fleet easily trackable.
However, it always required some sort of recalibration or extra scan to detect moving cloaked ships. I could never see, why it was always a bit of a break through to do that. One should think once you knew how to beat them ...
The detection of cloaked ship nets are all right ect, but there are few mistakes in that logic...
1st: klingons helping federation, as in dominion they had short war and the new cancelour wasnt a federation friend there is similar possibility that klingons could either help romulans or just attack federation to claim territory from a weak foe (like they did many times) i would actually say taht its 50/50 on that point.
2nd: you presume that romulans launch a whole fleet in 1 place... not logical as most engages are multi targeted to spread chaos, and as we had seen groups of single DD's can easy go undetected through these all defences (it just takes much longer to prepere). Also dont use the DS9 example as possibility to detect rom ships as they could only do that as O'brien knew from future visions that roms will detect and he was searching days to find them (why we dont know why roms were waiting afk). Also as we see in Nemesis the valdores and scimi are 100% non detectable to the most advanced fed ship in 1km range...
3rd: from what i read on alpha roms had lost around 40% of DD's what is a very good outcome considering federation losses. You have to also rememeber that DD's are utility ships (a huge nebula with all 3 pods in one x1,5) but cheaper to produce then galaxy and can probably cripple or destroy galaxy class ships in 1 on 1 run (like shown in TNG when a shot ALONE from main cannon take Galaxy class vessel shields to 30%). Damn it had enough power to actually whipe out a whole starbase. Yes they have weakness because of the huge canon shots for weaker ships but formation runs and cloak kinda pulls it up to some point (and the moment of surprise that in series never was used). Also the new Mogai seems more combat oriented, and it looks like its a version of DD that is pure combat oriented with similar tankyness but way more fire power (the 4 frontal disruptors - where roms didnt want to fire torps - have more effect then every torp and frontal phaser on Enterprise) and around 2x turn rate. So they dont have such little ship weaknes and augmenet the utility DD's very good and probably would hit sovereign class as hard in initial run as DD's did to Galaxy. To it there's also the problem that DD's and Mogais can be mass produced, they seem of similar power to fed TOP OF THE LINE but around 50% as hard to produce - such advantage wins wars.
4th: Federation couldnt even handle the short was vs klingons 1 on 1 where klinks had their army in cardassian space, i wouldnt even dare to say that feds survived Dominion or Klingons, as i wouldnt say they could survive Romulan invasion 1 on 1 (and their suriprise attack because of how spread their forces are - even if they have the largest fleet as they are the largest empire).
The reason why roms dont attack feds is probably is because roms still dont have definitive advantage and thus the outcome isnt rly shure and it could end up in a long and bloody war not beneficial, while there is also the 50/50 risk of who do klinks help. Also there still may be the problem of to low ship diversification (but well we have 2 heavy hitters of witch 1 is utility also but weaker, and scout ships and sci light detector from TNG).
to sum things up... yes cloak is broken OP invention and it shouldnt be made so good in TOS :D, that would make the power balancing for Serier easier and clearer.
